This invention relates to a yoghurt-making machine, inter alia for domestic use, enabling yoghurt to be prepared from different commercially available milks.
At present two kinds of yoghurt-making machines are commercially available. One of them comprises an isothermal enclosure in which earthenware pots are disposed. The milk, preheated to about 50.degree. C and seeded with lactic ferments is poured into the pots, and then the isothermic enclosure is closed. The milk ferments and the yoghurt is formed during cooling. This kind of yoghurt-making machine is very simple is design, but it has the disadvantage of requiring separate heating of the milk and checking of its temperature before performing seeding with lactic ferments.
The other kind of known yoghurt-making machine comprises electrical heating means. The cold-seeded milk is poured directly into a receptacle or into pots disposed in the enclosure. Then the milk is heated until thermal equilibrium is reached between the heat supplied by the electrical heating means, the heat absorbed by the yoghurt-making machine, and the heat dissipated to the atmosphere.
The disadvantages of the last-mentioned machines are that they operate slowly due to the low speed at which the temperature of the milk rises, and the temperature cannot be accurately controlled, as a result of fluctuations in the mains voltage and the ambient temperature. However, it is important to keep the milk at a predetermined constant temperature, so as to give the yoghurt a suitable degree of acidity and a sufficiently-developed aroma. Experience has shown that the optimum yoghurt-forming temperature is close to 45.degree. C, such temperature being favourable to the development of Streptococcus thermophilus, which acts on acidity, and Lactobacillus bulgaricus, which develops the aroma. It is also found that the aforementioned temperature should be maintained for at least two hours.